1. Department of Epidemiology, Graduate School of Public Health, University of Pittsburgh, Pittsburgh, PA 15261, USA
2. Department of Epidemiology, Michigan State University, 644 West Fee Hall, East Lansing, MI 48824, USA
3. Department of Pediatrics, University of Pittsburgh Medical Center, Pittsburgh, PA 15224, USA
4. Department of Microbial Pathogenesis, University of Maryland Dental School, Baltimore, MD 21201, USA
5. School of Public Health, The University of Texas Health Science Center, Houston, TX 77030, USA